Circuit for varying the hysteresis of a schmitt trigger



Oct. 21, 1969 R. s. HUGHES 3,474,264

CIRCUIT FOR VARYING THE HYSTERESIS OF A SCHMITT TRIGGER Filed June 16. 1966 2 Sheets-Sheet 1 5 UNSTABLE 0 ABOVE LOOSK o I l 1 970 980 990 L000 I,Ol0

INVENTOR. R RICHARD s. HUGHES JOHN M. KOCH ROY MILLER ATTORNEYS.

R. HUGHES Oct. 21, 1969 CIRCUIT .FOR VARYING THE HYSTERESIS OF A SCHMI'IT TRIGGER Filed June 16, 1966 2 Sheets-Sheet 2 l- E V F- L l- E R H I- G la I R R E T G G R I E I W w R E P /r P u n 5 e R R u 5 o FIG. 3.

INVENTOR, RICHARD S. HUGHES G o l K 0 In 8 R 6 5 4 E L B A 2 TOK SLI m NEG UBI ll O 5 0 O 0 o 6 4 2 JOHN M. KOCH ROY MILLER ATTORNEYS.

United States Patent Us. or. 307-490 1 Claim ABSTRACT OF THE DISCLOSURE A circuit for externally varying the hysteresis of a Schmitt trigger without changing the operating characteristics of the Schmitt trigger by alternately introducing and withdrawing resistance from the input circuit to vary the level of the input as seen by the Schmitt trigger.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

It is generally known that the conventional Schmitt trigger circuit exhibits hysteresis in that to effect a transition in one direction it is necessary to first pass beyond the voltage at which the reverse transition takes place.

There are many situations, such as in ramp and other function generators, where the hysteresis offered by the Schmitt trigger circuit could be utilized if the range of the hysteresis could be suitably widened in an efiicient straight-forward manner and without degradation of the trigger operation relative to the output rise time of the circuit, the output pulse amplitudes of the circuit, etc.

Accordingly, it is a principal object of this invention to provide a modified Schmitt trigger circuit wherein the range of the hysteresis is suitably widened for useful applications of the circuit, in an efficient manner, and without degradation of the desired trigger operation properties or qualities of the circuit.

Additional objects of the invention will become apparent from the following description, which is given primarily for purposes of illustration, and not limitation.

Stated in general terms, the objects of the invention are attained by providing circuit means externally of the Schmitt trigger circuit configuration for controlling the pulse amplitude of the input to the Schmitt trigger circuit, preferably predetermined resistance means that are alternately introduced into, and withdrawn from the circuit of either the upper trigger level, the lower trigger level, or at both trigger levels to widen the range of the hysteresis of the Schmitt trigger circuit in a desired manner without detrimentally affecting desired circuit operation characteristics.

A more detailed description of a specific embodiment of the invention is given below with reference to the drawings, wherein:

FIG. 1 is a schematic circuit diagram showing a modified Schmitt trigger circuit of the invention;

FIG. 2 is a graph showing the changes in the upper and lower trigger levels as a function of the resistor R FIG. 3 is a similar graph showing the effect of resistor s;

FIG. 4 is a graph showing the hysteresis as a function of small changes in the value of resistor R and 3,474,264 Patented Oct. 21, 1969 ice FIG. 5 is a similar graph showing the effect of resistor R As shown in FIG. 1, transistors Q and Q are connected in the customary Schmitt trigger circuit configuration. Externally of this configuration, transistor Q, is connected to transistor Q, the supply voltage V and the input terminal V to act as a buffer to avoid loading the driving circuitry. The Zener diode Z is connected to transistor Q, to protect the transistor against over-voltage effects. Transistor Q also is connected to resistor R and diode D to further modify the Schmitt trigger circuit externally thereof. A resistor R diode D and transistor Q; are connected in series in a line leading from the input terminal V to ground in a manner to control the pulse amplitude of the input to the Schmitt trigger circuit, along with resistor R diode D and transistor Q as described below.

In operation, in the normal state, transistor Q is off and transistor Q is on and assumed to be saturated. The voltage V at the base of transistor Q, is given approximately by the equation:

: S iu Re+ Rs This assumes no loading on resistance R by Zener diode Z or transistor Q and that transistor Q is at ground potential. Resistor R is not in the circuit. Transistor Q; is off, thus back-biasing diode D and removing resistor R, from the circuit, which includes R R D and Q When the voltage V nears the inherent upper trigger level of the circuit, transistor Q starts conducting. T ransistor Q continues to conduct until transistor Q turns off. This action turns on transistor Q At this point, diode D is back-biased and resistor R is removed from the circuit. Diode D becomes forward-biased thus placing resistor R into operation in a circuit including R R D and Q At this stage, the voltage V is given by the following equation:

Rs-l- 1 The lower trigger point of the circuit can be controlled by resistor R Zener diode Z is used to limit the maximum voltage across resistor R and to prevent forward biasing of the base-collector circuit of transistor Q During a cursory examination of the circuit of FIG. 1, it would appear than transistor Q can be used in the place of Q because the states of both transistors are essentially the same during the circuit operation. However, the problem resides in the fact that transistor Q functions as a linear amplifier for a short time before transistor Q switches on. Also, it is a difficult problem to arrange to have the collector voltage of transistor Q to approach the value of the ground potential.

The inherent upper trigger point for the Schmitt trigger circuit, including transistor Q, in the circuit, is designated voltage X in the following equation, and the lower trigger voltage point is designated Y. From Equation 1, one obtains the following equation for V at the upper trigger point:

in and at the lower trigger point, one obtains for V e+ R1) vi..=

The difference between the values of V at the upper trigger point and at the lower trigger point is the hysteresis H:

FIGS. 2 and 3 show the effects of R and R respectively, on the upper and lower trigger levels of the modified Schmitt trigger circuit of the invention shown in FIG. 1. It will be seen that the hysteresis can be controlled and widened by the use of the variable hysteresis Schmitt trigger circuit of the invention. FIGS. 4 and 5 show the hysteresis of the circuit as a function of small changes in the values of resistors R and R respectively.

It will be seen that the variable hysteresis Schmitt trigger circuit of the invention can be used with only a low trigger point controlling resistor, such as R only a high trigger point controlling resistor, such as R or two resistors, such as R and R for controlling both the upper and lower trigger levels of a Schmitt trigger circuit. Also, it will be understood that vacuum tube amplifiers, as well as transistor amplifiers, can be used in the modified Schmitt trigger circuit of the invention.

Obviously, many other modifications and variations of the variable hysteresis Schmitt trigger circuit of the invention are possible in the light of the teachings given hereinabove. It is therefore to be understood that the invention may be practiced otherwise than as specifically described and illustrated hereinabove.

What is claimed is:

1. A circuit for varying the hysteresis of a Schmitt trigger comprising:

an input;

a transistor having emitter, collector and base electrodes;

a first resistance coupled between said input and the input to a Schmitt trigger;

a series combination of a second resistance and a first diode operatively connected between the input to the Schmitt trigger and the collector of the transistor wherein,

the cathode of the first diode is connected to the collector of the transistor;

a third resistor operatively connected between the collector of the transistor and a biasing voltage;

means for grounding the emitter of the transistor;

a fourth resistor coupled between the base of the transistor and the output from the Schmitt trigger;

a series combination of a fifth resistor and a second diode operatively connected between the input to the Schmitt trigger and the output from the Schmitt trigger wherein,

the cathode of the second diode is connected to the output of the Schmitt trigger; and

a sixth resistor operatively connected between the base of the transistor and ground.

References Cited I UNITED STATES PATENTS 2,923,840 1960 Ellsworth 307-290 2,986,650 1961 Wolfendale 307-290 3,151,256 1964 Simon et a1. 307-290 3,302,037 1/ 1967 Neurnann 307290 DONALD D. FORRER, Primary Examiner H. A. DIXON, Assistant Examiner US. Cl. X.R. 

